The eruption of the Shinmoedake volcano hundreds of miles away may be linked to the quake, but it's difficult to prove, one scientist says. In general, though, one can easily affect the other.

The Shinmoedake volcano on Japan's southern island of Kyushu, 950 miles from the epicenter of last week's magnitude 9 earthquake, spewed ash and rocks up to 2.5 miles into the air March 13. The volcano had erupted Jan. 19 and several times afterward, most recently Feb. 1. Its re-eruption just two days after the massive temblor prompted many to wonder whether the quake could have triggered that event.

"The last explosion event at Shinmoedake may be triggered by the shock of the earthquake," said Setsuya Nakada of the Earthquake Research Institute at the University of Tokyo in an e-mail. "Nobody can verify the relationship, however."

Though scientists may not know all the facts in this case, there is a relationship — at least in principle — between earthquakes and volcanoes. If an earthquake starts out large enough, it can certainly trigger a volcano many hundreds of miles away, said UC Santa Barbara volcanologist Frank Spera.

"These things are kind of like snakes in a box. They're ready to go, so you wouldn't need a huge push to get it going," Spera said. "An earthquake of this magnitude could provide that. It's not outlandish.... It's like taking a hammer and just whacking a large bell; there's going to be vibrations. The bell will continue to ring for a while."

Though it doesn't appear to happen often, an earthquake may activate a volcano in several ways:

• The energy waves traveling through the ground may break up the hardened top of the volcano's magma reservoir, allowing the molten rock to escape, said John Ewert, a volcanologist with the U.S. Geological Survey. These usually cause what looks like a "leakage," Ewert said. The 1975 Kalapana earthquake in Hawaii caused the Kilauea volcano to flow in that manner.

• The earthquake causes a shifting around of gas in the volcano's magma that may trigger an explosive eruption. Think of a bottle of soda pop, Ewert said: "If you give things a shake and you release bubbles, those bubbles can rise to shallower areas where they're under less pressure."

Stress caused by earthquake can squeeze magma reservoirs, pushing the magma to the surface.

The lower the reservoir of molten rock beneath the volcano, the lower the chance of this. That's because deep beneath the surface — say, 25 miles — the Earth's crust has fewer cracks and fissures to be disturbed.

This is what should happen, volcanologists say. But observing it in practice is often hard to do. Of the more than 1,500 potentially active volcanoes on Earth,only 50 to 100 have multiple sensors on them to enable scientists to know what's happening beneath the volcano's surface, Ewert said.

If earthquakes spawned eruptions with any degree of predictability, an eruption would have been highly likely after the 2004 magnitude 9.1 to 9.3 Sumatra earthquake, which devastated Indonesia. Indonesia has 129 active volcanoes. (Japan has 106 active volcanoes). Afterward, not a single quake-associated eruption occurred.

On the other hand, two days after Chile's 9.5 earthquake in 1960 — the largest earthquake of the 20th century — the Cordon-Caulle volcano in the southern region of the country erupted after four decades of inactivity.

The bottom line: The relationship between earthquakes and volcanic eruptions is confusing but exists. The relationship can work both ways: When a volcano erupts, the movement of magma beneath the surface can cause rocks to break, triggering tremors that appear similar to earthquakes caused by shifting tectonic plates. But these tremors are generally small, below magnitude 5, and within 20 miles of the eruption.

As to whether the recent activity of the Shinmoedake volcano was a harbinger of the giant quake March 11, there's no reason to suppose that, scientists said.